See explanation below. Explanation You need to create a customer lookup field then add it to the Opportunity entity. * Go to Settings > Customizations. * Select Customize the System. * Under Components, expand Entities, and then select the Opportunity entity. * Select Fields. * Click New. * Create the Billing Contact field. * Set the Field Requirement to Business Required. * Set the Data Type to Lookup. * For the Target Record Type select Contact. * Click Save and Close. * Click Publish All Customizations. * Open the Opportunities form. * In the Field Explorer drag the new field into the form. * Click Save * Click Publish